Actuators have come a long way since the invention of rotary motors, which set the foundation for robotics and marks the dawn of the age of automation and industrialization. The drastic improvement in performance of hard actuators nowadays is only matched by the large number of emerging soft actuators, which demonstrate functionalities tantamount to or more expansive than that of their hard counterparts.
Robots or machines capable of complex movements often require many actuators working in synchrony. Such systems are potentially difficult to control. One way of reducing the complexity in control is to have parallel actuation in the system, where one or a few inputs can result in many outputs working synchronously in a desired way. For hard machines, parallel actuation can be realized through gears and levers in high precision. In soft machines, however, the counter parts of such parallel actuation systems are rare or non-existent.
“Muscle” is the almost universal actuator in animals. In efforts to mimic aspects of the mechanics of (if not the mechanism of action of) biological muscle, a large range of synthetic structures has been explored but none has successfully replicated the essential features of muscle. Muscle has three features that have remained difficult to replicate: muscle i) maintains roughly a constant volume upon contraction; ii) shows a useful compromise between speed of actuation, and force applied during actuation; iii) has mechanical properties (e.g., stiffness and density) that are compatible with the requirements of animals.
McKibben actuators, developed in the 1950s, were examples of muscle-mimetic structures. These actuators comprise a rubber balloon, surrounded by a fiber-reinforcing mesh. On pressurization, the balloon inflates anisotropically (with a motion that reflects the structure and mechanics of the surrounding mesh), and this expansion results in useful motion. The properties of the fiber reinforcement (i.e., the density of the weave, and the strength of the fibers) dictate the strain (typically 25%) and load (typically 80˜130 N/cm2) the actuator can produce for a given pressure. McKibben actuators have been used in many practical applications, but suffer from two major disadvantages: 1) their inherit dry friction creates heating (which changes the properties of the actuator) and hysteresis that makes precise positional control difficult; 2) they become stiff and their specific tension decreases (and approaches zero at ˜25% strain) as the actuators shorten.
Soft actuators are important for their ability to contact delicate, soft, and irregularly shaped objects (i.e., humans and animals, fruit, produce), because they distribute forces across the surface of the objects, and because they are fabricated of compliant rather than unyielding materials and structures. They also offer an attractive approach to simplifying controls, since they make it possible—in some circumstances—to substitute the properties (and especially non-linearities such as “snap-through”) of materials and structures for some of the control loops, sensors, and actuators of hard robots.
Pneumatically actuated soft machines are being actively developed, but, as a clan, they have two characteristics that can limit their use in some applications: i) they can burst when over-pressurized, and therefore may be dangerous or unreliable when used outside their specified operational ranges; ii) most increase in volume when pressurized, and thus cannot be used in applications in confined spaces. Thus, there remains a need for new and more effective actuators.